If you have vellus under your arms and on you scrotum and at the base of your penis, have you hit puberty?
People have vellus hair covering their whole body, except for the palms of their hands and the soles of their feet. These hairs are there your whole life. The picture to the right shows vellus hairs on a person's arm.As you develop, some of these vellus hairs change into terminal hairs. They first get longer and then start developing color. Often vellus hairs, even when long, are not easily seen unless light shines across them sideways. Notice in the picture that the hairs on the left have gotten long, but haven't taken on much color yet. The hairs on the right have become terminal hairs.
So the fact that you have vellus hair doesn't mean you have reached puberty. However, since you are noticing the hairs, they might have started growing but not yet become tinted.
Typically the first noticeable sign of puberty is the swelling of the scrotum, but for most boys the change is so subtle that they don't notice. Usually it is spontaneous erections that are the first clue to a boy that he has passed puberty.